OP here– good point, that's not well explained in the article! And thanks to the others who have answered already, you're pretty much spot on.

The core library, referenced here as `el`, contains a set of factory functions for describing various underlying processing nodes. So when you write `el.lowshelf()` you're just using one of the builtins (like how you might use a div, span, or button in a web app with React)